Imac Headphone output stopped working

Assuming that the jack was working fine before, it may be the jack itself. Have you tried other headphones just to determine if the plug is oddly sized which can happen.

 

It could be the jack socket is loose on the logic board. If you jiggle the headphone plug while in the socket (gently) from front to back, do you hear scratching or the sound dropping in and out? Have music playing while you try this step.

If it is, then you have a loose connection on the board itself which could be due to stress, dry solder joints, etc. If you are still under warranty or Apple care you can have it repaired.

 

So when you plug in the headphones you hear the music at normal volume i.e. loud enough to be clear, in the left side briefly?

With them plugged in, have you played with all the settings for sound in the pref panel? .e.g change the sound output to something else then back. (if you have that option)

Definitely sounds odd.

 

One thing worth trying, do a PRAM reset for a minimum of 3 'dongs'.

Just in case you're not familiar with that process:

Resetting PRAM and NVRAM

1. Shut down the computer.
2. Locate the following keys on the keyboard: Command, Option, P, and R. You will need to hold these keys down simultaneously in step 4.
3. Turn on the computer.
4. Press and hold the Command-Option-P-R keys. You must press this key combination before the gray screen appears.
5. Hold the keys down until the computer restarts and you hear the startup sound for the second time.
6. Release the keys.

Repeat this 2 more times so that you have heard the machine startup 3 times. This may well clear the sound problem. Let me know if you have success.

 

Sorry I wasn't able to help with your problem. Might be time to take your iMac to Apple for a hardware check.

 

Because it is the headphone port a SMC reset may help. Unplug all power from the machine and hold the power button in for about 10 seconds, then plug everything back in and startup the iMac. It's the "one last test" before seeking Genius/professional help at a service center.